Magnetic resonance imaging (MRI) is a new diagnostic modality that involves exposing subjects to static magnetic, gradient magnetic, and radiogrequency magnetic fields; each of which has the potiential to produce adverse biological effects. To date, there have been no investigations of the possible health risks associated with MRI. The U.S. Food and Drug Administration has issued guidelines for the safe use of MRI, however, we hypothesize that these recommendations are unnecessarily restrictive. The primary unsafe biological effects of MRI are related to the thermogenic qualities of the electromagnetic fields that are required for this imaging procedure. Therefore, the focus of this investigation will be to examine the alterations in temperature and thermophysiological responses produced by operating a highfield (1.5 Tesla) MRI system below and above the recommended guidelines. The specific aims of this research are: to determine the temperature changes and thermophysiological responses produced by clinical MRI of normal subjects, patients with normal thermoregulation capabilities, and patients with conditions associated with heat intolerance (i.e. cardiovascular disease, diabetes, obesity, etc.); to determine the temperature changes of thermal sensitive tissues (i.e. testis, eye) produced by clinical MRI; to determine the effect of body composition on the temperature changes and thermophysiological responses produced by clinical MRI; to determine the possible presence of "hot spots" produced by clinical MRI; to determine the effect of the static magnetic field on body temperature; and to determine the subjective perceptions of patients to clinical MRI. The methods will include: measurement of oral, esophageal, skin, superficial fat, muscle, scrotal and corneal temperatures using appropriated techniques; measurement of heart rate, blood pressure, and respiratory rate; determination of skin blood flow by laser- Doppler velocimetry; assessment of body composition by skin-fold calipers and bioelectrical impedance plethysmography; determination of heating patterns by high-resolution thermography; and assessment of patient subjective perceptions by means of a standardized postMRI questionnaire. Approximately 500 patients will be studied and a computerized data base will be established to facilitate data retrieval and analysis. The resulting information will provide significant and important data to help establish the safe level of operation of MRI systems and will also contribute to the understanding of the biological effects associated with the unique combination of electromagnetic fields used for this type of diagnostic imaging.